The present invention relates to apparatus for dynamically compensating, at least in part, polarization dispersion in an optical fiber transmission system. A long-haul optical fiber transmission system comprises:
a transmitter terminal essentially constituted by a laser diode transmitting a completely polarized optical signal;
a monomode optical fiber conveying the signal transmitted by the transmitter terminal; and
a receiver terminal receiving the optical signal conveyed by the fiber.
All types of fiber suffer from polarization dispersion: a pulse transmitted by the transmitter terminal is received distorted. Its duration is greater than its original duration. Such distortion is due to the fact that the optical signal becomes depolarized while it is being conveyed: the signal received at the end of the link fiber may be considered to be constituted by two orthogonal components, one of which corresponds to a polarization state for which the propagation speed is at its maximum (fast main polarization state), the other component corresponding to a polarization state for which the propagation speed is at its minimum (slow main polarization state). In other words, a pulse signal received at the end of the link fiber can be considered to be made up of a first pulse signal polarized in a privileged polarization state, and arriving first, and of a second pulse signal propagating in a delayed propagation state, and arriving with a delay referred to as the xe2x80x9cinstantaneous differential delayxe2x80x9d which depends in particular on the length of the link fiber.
If the transmitter transmits an optical signal made up of a very short pulse, the optical signal received by the receiver terminal is made up of two successive pulses that are polarized orthogonally and that have a time offset equal to the instantaneous differential delay. This delay may be about 20 picoseconds for a link that is 100 kilometers long and that is made using monomode fiber of the kind manufactured a few years ago. The distortion of the pulses received by the receiver terminal can cause errors in decoding the transmitted data. Therefore polarization dispersion constitutes a limiting factor on the performance of optical links, both analog links and digital links.
It is now known how to manufacture monomode fibers having low polarization dispersion (about 0.05 picosecond/km). However, a large fraction of monomode fibers that have been installed over the last ten years have very high polarization dispersion, and that constitutes a major technical obstacle to increasing transmitted data rates. In addition, the same problem may well arise again even with low polarization dispersion fibers, assuming the race for ever higher data rates continues.
It is known how to make high polarization dispersion fibers that make it possible, by using short segments, to procure a fixed differential delay. Such fibers are also referred to as xe2x80x9cpolarization-maintaining fibersxe2x80x9d. By appropriately disposing such a component (or any apparatus for generating a differential delay between two orthogonal polarization modes) in series with a transmission link having high polarization dispersion, it is possible to compensate the polarization dispersion optically. That may be achieved either by using a polarization-maintaining fiber having the same differential delay as the link, but with the slow main polarization state and the fast main polarization state being swapped over, or by causing a main polarization state of the assembly constituted by the link and the polarization-maintaining fiber to coincide with the polarization state of the transmission source. To that end, a polarization controller is used that is placed between the link and the polarization-maintaining fiber.
The value of the differential delay and the main polarization states of a link vary over time as a function of many factors, such as vibration and temperature. Compensation apparatus must therefore be adaptive, and the differential delay of the polarization-maintaining fiber must be chosen to be not less than any of the differential delay values that are to be compensated.
U.S. Pat. No. 5,473,457 describes a method and apparatus for compensating polarization dispersion in an optical transmission system. That apparatus comprises:
a polarization controller and a segment of polarization-maintaining fiber interposed between the link fiber and the receiver terminal, in that order;
servo-control apparatus controlling the polarization controller as a function of an error signal;
means for modulating the frequency of the laser constituting the source of the transmitter terminal; and
apparatus for measuring the intensity modulation of the received signal, which apparatus comprises a polarizer splitting the signal delivered by the polarization-maintaining fiber into two orthogonally-polarized signals, the axis of the polarizer being disposed at an angle of 45xc2x0 relative to the intrinsic axes of the polarization-maintaining fiber.
The polarization controller makes it possible to steer the polarization of each of the components of the optical signal delivered by the link fiber, the polarization being steered by turning it through a given angle defined by the value of a command signal applied to the servo-control apparatus. The servo-control means implemented to compensate the polarization dispersion serve to align the slow main state of the link to be compensated with the fast main state of a polarization-maintaining fiber and to align the fast main state of the link to be compensated with the slow main state of said polarization-maintaining fiber which is selected so that its differential delay coincides with the mean polarization dispersion of the link to be compensated. The frequency modulation of the optical signal is converted into polarization modulation by the polarization dispersion of the assembly constituted by the link fiber and by the polarization-maintaining fiber, and then into intensity modulation by the polarizer situated in the apparatus for measuring the intensity modulation of the received signal. The depth of the intensity modulation is directly proportional to the instantaneous differential delay generated by the polarization dispersion, and as weighted by the impact of the polarization of the transmission source (effective differential delay). The intensity modulation signal is used as an error signal to control the polarization controller which is inserted between the link fiber and the polarization-maintaining fiber, the polarization controller being controlled in such a manner as to minimize the intensity modulation signal, and thus to minimize the effective differential delay produced by the assembly comprising the link and the polarization-maintaining fiber. Ideally, the polarization dispersion of the link fiber is exactly compensated by the polarization dispersion, of opposite sign, created by the polarization-maintaining fiber.
That known apparatus suffers from the following drawbacks:
not only does it require the transmitter terminal to be modified to modulate the frequency of the optical signal, but it also requires the receiver terminal to be modified in order to compensate the polarization dispersion; and
it needs to be optimized specifically for each link to be compensated even when the link does not have different polarization dispersion.
An object of the invention is to provide optical apparatus that is situated entirely at the receiver terminal so that no modification needs to be made to the transmitter terminal, and that does not need to be optimized specifically for each link.
The invention provides apparatus for compensating polarization dispersion in an optical transmission system comprising a transmitter terminal for transmitting a polarized optical signal, a link optical fiber, optional optical amplifiers, and a receiver terminal, the apparatus comprising:
a first polarization controller;
means for generating a differential delay between two orthogonal polarization modes, said controller and said means being interposed between the link fiber and the receiver terminal, in that order; and
servo-control means for servo-controlling the first polarization controller;
said apparatus being characterized in that the servo-control means comprise means for steering the vector of the main polarization state of the assembly constituted by the fiber, the first polarization controller and the means for generating a differential delay, in particular so as to cause said vector to coincide with the vector of the polarization state of the signal at the output of the transmitter terminal.
The apparatus characterized in this way does not need any modification to be made at the transmitter end, and it enables polarization dispersion to be compensated completely provided that the differential delay produced by the compensator is not less than the differential delay existing in the link that is to be compensated. This apparatus thus offers the advantage of not needing to be optimized specifically for each link to be compensated even when the link does not have different polarization dispersion.
In a particular embodiment, the servo-control means comprise means firstly for measuring the degree of polarization of the optical signal delivered by the means for generating a differential delay, and secondly for controlling the first polarization controller in a manner such that the measured degree of polarization is maximized.
The apparatus characterized in this way makes it possible to servo-control the first polarization controller on two operating points. In particular, the servo-control means characterized in this way constitute means making it possible, at one of the two operation points, to steer the vector of the main polarization state of the assembly (constituted by the fiber, the first polarization controller, and the means for generating a differential delay) to cause said vector to coincide with the vector of the polarization state of the signal at the output from the transmitter terminal.
At both of the operating points, operation is based on the correlation existing between the polarization dispersion to which a signal is subjected while it is being conveyed, and the depolarization of the signal; The depolarization is measured by determining the degree of polarization of the received signal, i.e. the ratio of the power of the component in the main polarization direction of the signal divided by the total power of the signal.
Regardless of which operating point is chosen, measuring the degree of polarization of the received signal does not require any modification to be made to the optical signal transmitted by the transmitter terminal, nor does it require any particular means to be provided at the transmitter terminal. Therefore, the entire apparatus for compensating polarization dispersion may be situated at the receiver terminal, or for particular optimization reasons, anywhere else on the link, independently of the transmitter terminal. In addition, it is possible to dispose a plurality of compensation apparatuses along the transmission line.